WORM SHAFT BEARING ARRANGEMENT

Abstract: A worm gearbox comprising a one-piece casing (1) closed by a cover (2). A gearwheel (3) is mounted in the housing for rotation about a first axis and a worm shaft (4) meshes with the gearwheel and is mounted in a pair of bearing assemblies (5) for rotation about a second axis at 90 degrees to the first axis. A portion of the worm shaft (4) protrudes from one side of the casing (1) through a side aperture (8). Each bearing assembly (5) comprises a bush which is preferably made of a sintered metal material and wholly received in the casing, the bush being located therein so that all the radial and axial worm shaft loads are carried by the one-piece casing, the cover (2) retaining all the components in the casing. Preferably, the side aperture (8) is shaped to allow a sub-assembly including the worm shaft (4) with the bearing assemblies (5) fitted thereto to be inserted into the casing in a direction at 90 degrees to the axis of rotation of the worm shaft.

Description: WORM SHAFT BEARING ARRANGEMENT This invention relates to a worm gearbox used mainly but not exclusively for valve actuators and more particularly to the worm shaft bearing arrangement therein. In known worm and wheel gearboxes, the worm shaft normally has a portion which protrudes laterally from the gearbox casing through a circular aperture in the casing wall with a bearing bush therein. The other end of the shaft is located in a bearing cavity formed in the casing so the shaft is supported in bearings at two locations on either side of the worm drive where it meshes with the gear wheel. To assemble a worm gearbox of this type, the tubular bush first has to be fitted in the circular hole in the casing sidewall and the worm shaft can then be fed through this bush into the casing. Once inside, the thrust washers and worm can be mounted on the shaft and the free end of the shaft is then pushed into the end bearing cavity formed in the casing. It will be appreciated that this assembly process is very labour intensive and requires considerable skill to do it. The problem to be solved by the present invention is to design the worm gearbox in such a way that it is cheaper to manufacture, easier to assemble and thereby requires a lesser degree of labour skill for assembly. A worm gearbox is known which comprises a one-piece casing closed by a cover, the casing housing a gearwheel mounted therein for rotation about a first axis, a worm shaft meshing with the gear wheel and being mounted in bearing assemblies for rotation about a second axis at 90° to the first axis, a portion of said worm shaft protruding from one side of the casing through a circular side aperture therein. The present invention provides an improvement over the prior art by forming the casing in one-piece with each bearing assembly comprising a bush which is wholly received in the casing and located therein so that all the radial and axial worm shaft loads are carried by the one-piece casing, the side aperture in the casing being open on one side and the cover retaining all the components in the casing when fitted thereto. The side aperture in the casing being open on one side is an important feature of the invention as it allows a sub- assembly including the worm shaft with the bearing assemblies fitted thereto to be inserted into the casing in a direction at 90° to the axis of rotation of the worm shaft. Conveniently the side aperture is substantially square in cross section. With this side aperture configuration, it will be appreciated that it is no longer necessary to feed the worm shaft laterally through the side aperture in the casing wall for assembly purposes as it can instead be lowered into the casing which substantially simplifies the assembly process. Preferably each bearing bush is wholly received in a respective bearing cavity in the casing. In the preferred embodiment, the casing has a base with a rebate formed therein shaped to receive and locate one end of each bearing bush. Preferably each bearing bush comprises a body with an aperture therethrough to receive the worm shaft, said body having rounded ends and a plane bottom which fits into said rebate in the base and is located thereby. In the preferred embodiment, the casing includes rounded shoulder portions which cooperate with the rounded ends of each bearing bush body to take any radial and axial loads generated, in use, by the worm shaft. Preferably the rounded shoulder portions on the casing and the rebate in the base thereof are formed by machining the casing in a direction at 90° to the axis of rotation of the worm shaft. In the preferred embodiment, a pair of rounded shoulders are provided on the casing at one side only of each rebate formed therein. Preferably each rebate is oval when viewed in plan and of the same shape as the cross section of the body of the bearing bush but very slightly larger so that the bearing bush is a sliding fit therein. An oil seal is preferably fitted to the worm shaft and received in said side aperture to make an oil tight seal therewith. In the preferred embodiment, the gear wheel is a quadrant with teeth around the arcuate perimeter thereof but the wheel could be circular with teeth extending some or all of the way round its periphery. The bearing bushes are preferably made of a sintered metal material. According to a further aspect of the invention there is provided a method of assembling a worm gear box of the type specified comprising the steps of mounting the bearing assemblies, thrust bearings and an oil seal on the worm shaft to form a sub-assembly, lowering said sub-assembly into the casing in a direction at 90° to the axis of rotation of the worm shaft and fitting the cover plate to the casing to retain all the components therein. It is a further feature of the present invention that the one-piece casing has cavities formed therein which are machined in the casing in one direction only to receive and locate the bearing assemblies therein. Forming the bearing locating cavities in this way substantially cuts down the manufacturing time and therefore costs because it means that any machining of the casing only needs to be done in one direction whereas in the prior art, machining had to be done in several directions involving separate machining operations. Using sintered metal bearing bushes to support the worm shaft is also an important feature of the preferred embodiment of the invention because again it reduces the amount of machining required. It also means that rounded bearing surfaces can be provided on the body of the bearing assemblies which can cooperate with corresponding easily machined rounded bearing surfaces provided on the casing. A preferred worm gearbox of the present invention and method of assembly thereof will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a worm gearbox of the present invention; Figure 2 is a partial cross section of the worm gearbox shown in Figure 1; Figure 3 is a top plan view of the one-piece casing of the worm gearbox shown in Figures 1 and 2; and Figure 4 is a partial cross section of the worm gearbox shown in Figures 1 and 2. Referring to the drawings and particularly to Figures 1 and 2, there is shown a worm gearbox of the present invention which is intended for use as a valve actuator. It could however be used as an industrial gearbox in applications such as conveyor systems, pumps, mixers, lifting equipment etc. The gearbox comprises a one-piece casing 1 in which a gear wheel 3 is mounted for rotation about a first axis. The illustrated gear wheel 3 is formed as a quadrant and is provided with teeth 3a around its perimeter. The gear wheel is formed with flat ends which cooperates with stop screws 12,13 fitted into the casing 1 (only one flat end 12a is visible in Figures 1 and 2). The gear wheel 3 includes a hub portion 3d protruding laterally from each side thereof (only one such portion is visible in Figure 1) which is mounted in known manner for rotation about a first axis in bearing apertures 3c and 3e provided respectively in the casing 1 (see Figure 3) and cover plate 2 (see Figure 1) . The hub portions 3d and the gear wheel 3 have an axial bore 3b extending through them which receives in the direction of arrow A the actuator shaft of an appliance such as the stem or spindle of a valve (not shown). An end cap 10 is a press fit on the end of the hubs 3d protruding from aperture 3e in the cover plate 2 and rotates therewith. It can however be attached in other ways. An O-ring oil seal 11 is provided between the gear wheel 3 and the casing 1 (see Figure 4). An O-ring oil seal (not shown) can also optionally be fitted between the hub 3d and the indicator cap 10. A worm shaft 4 with a worm gear 9 thereon is fitted in the casing 1. A handwheel (not shown) can be fitted to the end of the shaft 4 protruding from the casing 1 or it can be connected to some other means for rotation. The worm 9 meshes with the teeth 3a on the quadrant gear 3. The shaft 4 is formed with a shoulder 15 adjacent both ends of the worm 9 for reasons which will be explained shortly. The casing is provided with an aperture 8 at one side thereof which is square in cross section as can be seen from Figures 1 and 4. It will be noted from Figure 4 that the opening 25 is not wholly enclosed by the side wall of the casing 1 but instead has an open side 25a which is closed by the cover plate 2 after assembly on the casing 1 for reasons which will be explained. An oil seal 8 located in the opening 25 in known manner. The worm shaft 4 is mounted in a pair of bearing bush assemblies 5 (only one is visible in Figures 1 and 2). Each bearing bush assembly has a central hole in it which receives the shaft 4. The body of the bush has rounded, preferably semi-circular, ends 20 and flat or plane upper and lower surfaces 21. These bearing bushes 5 are made of a sintered metal material. The casing 1 has a perimeter wall la in which the side opening 25 is formed. It also includes oval shaped rebates 22 in the base thereof which, by way of example, are approximately 3mm in depth. These rebates 22 receive the flat bottom 21 of each bearing bush 5 and locates it therein. The side wall la of the casing wall is also formed with arcuate bearing surfaces 23 which are quadrant shaped as shown in Figure 3 and cooperate with the rounded ends 20 of the bearing bushes 5 whereby all radial and axial loads generated by the shaft 4 are carried by the casing 1. A thrust bearing 7 is sandwiched between a pair of thrust washers 6 located between each annular shoulder 15 on the worm shaft 4 and its adjacent bearing bush 5 (only one such assembly is illustrated in Figures 1 and 2 - the other assembly is identical and a mirror image of the illustrated assembly) . In order to assemble the worm gearbox illustrated in the drawings, a sub-assembly is first made up comprising worm shaft 4 on which a first thrust washer 6 is mounted to abut the right hand shoulder 15. A thrust bearing 7 and a second thrust washer 6 are then mounted on the shaft 4 followed by the bearing bush 5 and the oil seal 8. A second thrust bearing 7 sandwiched between a pair of thrust washers 6 is then fitted on the front left hand end of the shaft (not visible in Figures 1 and 2) to abut the shoulder 15 adjacent the front left hand end of the shaft 4. The sub-assembly is now ready for insertion into the casing 1. Conveniently, the thrust bearings 7 are needle bearings but other thrust bearings can be used. Because the side aperture 25 is open on one side, it will be appreciated that the bearing bushes 5 forming part of the whole sub-assembly can be lowered into the casing 1 so that the bottoms 21 fit into and are located in the rebates 23, the rounded ends 20 of the bushes 5 locating against bearing surfaces 23 formed in the casing 1. The oil seal 8 fits into the aperture 25. The cover plate 2 can then be fitted to the casing 1 and secured thereto by means of the bolts 14 thereby retaining the bearing bushes 5, the oil seal 8 and all the other components in position in the casing as well. Because of the particularly novel construction of the casing of the present invention, it will be appreciated that once the worm shaft 4, two bearing bushes 5, two thrust bearings 7 and associated pairs of thrust washers 6 and oil seal 8 have been assembled into a sub-assembly, this can then be readily slid into the casing 1. Thus, the time consuming prior art process is avoided of having to feed the worm shaft 4 laterally through a circular aperture in the casing side wall and then having to fit all the various components to the shaft before locating its free end in a cavity in the facing casing wall. Because of the novel construction of the casing of the invention, it will be appreciated that the machining operations needed to form the bearing surfaces 23 in the casing can be carried out only in one direction, i.e. at 90° to the axis of rotation of the worm shaft 4. This can be done in a simple milling operation. Because the bearing bushes 5 are made of a sintered metal material, they can be moulded to provide the cooperating rounded bearing surfaces 20. As a result, the manufacturing process is substantially simplified as is the assembly of the worm gearbox because all the machining steps in the manufacturing process of the casing are done in one direction and the assembly can take place by simply dropping the sub-assembly into the casing and holding it in place with the cover 2. If the worm gearbox of the invention is to be used, for instance, to operate a quarter-turn valve, face lb of casing 1 is bolted to the valve housing (not shown) with the valve spindle (not shown) received in the bore 3b and drivingly keyed thereto. Thus, when shaft 4 is rotated, e.g. using a handwheel (not shown) connected thereto, the worm 9 rotates the gear wheel 3 and the valve spindle (not shown) drivingly connected thereto to " open or close the valve.

Claims: CLAIMS 1. A worm gearbox comprising an one-piece casing closed by a cover, the casing housing a gearwheel mounted therein for rotation about a first axis, a worm shaft meshing with the gearwheel and mounted in bearing assemblies for rotation about a second axis at 90 degrees to the first axis and having a portion protruding from one side of the casing through a side aperture therein characterised in that the casing is in one-piece and each bearing assembly comprises a bush which is wholly received in the casing and located therein so that all the radial and axial worm shaft loads are carried by the one-piece casing, the cover retaining all the components in the casing. 2. A worm gearbox as claimed in claim 1 wherein the side aperture is shaped to allow a sub-assembly including the worm shaft with the bearing assemblies fitted thereto to be inserted into the casing in a direction at 90 degrees to the axis of rotation of the worm shaft. 3. A worm gearbox as claimed in claim 1 or claim 2 wherein each bearing bush is wholly received in a respective bearing cavity in the casing. 4. A worm gearbox as claimed in claim 3 wherein the casing has a base with a rebate formed therein shaped to receive and locate one end of the bearing bush. 5. A worm gearbox as claimed in claim 4 wherein the bearing bush comprises a body with an aperture therein to receive the worm shaft, said body having rounded ends and a plane bottom which fits into said rebate in the base and is located thereby. 6. A worm gearbox as claimed in claim 5 wherein the casing includes rounded shoulder portions which cooperate with the rounded ends of each bearing bush body. 7. A worm gearbox as claimed in claim 6 wherein the rounded shoulder portions on the casing and rebate in the base thereof are formed by machining the casing in a direction at 90 degrees to the axis of rotation of the worm shaft. 8. A worm gearbox as claimed in claim 6 or claim 7 wherein a pair of rounded shoulders are provided on the casing at one side only of each rebate formed therein. 9. A worm gearbox as claimed in any preceding claim wherein the side aperture is substantially square in cross section. 10. A worm gearbox as claimed in claim 6 wherein the rebate is oval when viewed in plan and of the same shape as the body of the bearing bush. 11. A worm gearbox as claimed in claim 9 wherein an oil seal is fitted to the worm shaft and received in said side aperture to make an oil tight seal therewith. 12. A worm gearbox as claimed in any preceding claim wherein the gearwheel is a quadrant with teeth around the arcuate perimeter thereof. 13. A worm gearbox as claimed in any preceding claim wherein the bearing bushes are made of sintered metal. 14. A method of assembling a worm gearbox of the type claimed in claim 1 comprising the steps of mounting the bearing assemblies thrust bearings and an oil seal on the worm shaft to form a sub-assembly, lowering said sub- assembly into the casing in a direction at 90 degrees to the axis of rotation of the worm shaft and fitting the cover plate to the casing to retain all the components therein. 15. A one-piece casing for a worm gearbox as claimed in claim 1 in which cavities have been machined in the casing in one direction only to receive and locate the bearing assemblies therein. 16. A worm gearbox substantially as herein described with reference to the accompanying drawings.

Inventors: HARVEY RAYMOND GODFREY ASHLEY, HARVEY, RAYMOND, GODFREY, ASHLEY
Applicants: HARVEY RAYMOND GODFREY ASHLEY, OPPERMAN MASTERGEAR LTD, HARVEY, RAYMOND, GODFREY, ASHLEY, OPPERMAN MASTERGEAR LIMITED
Application year: 1994
Main classification: F16H57
IPC codes: F16H1, F16H57